The spliceosome has to be one of the strangest words — and wondrous wizards — of evolution. Consider this. Messenger RNA, which copies the protein-building message from the genes and then ferries it to the cell's assembly plant, is an edited version of the genetic blueprint. Or to put it another way, somewhere along the line from copying to assembly, the messenger somehow deletes part of the message, specifically those stretches of non-coding DNA, sometimes called "junk DNA" or "introns." How could messenger RNA accomplish this feat of pinpoint accuracy? As it turns out, it doesn't. That task falls to a molecular splicing machine — the spliceosome — that literally snips out the introns and reconnects the key coding sections.

In the early 1980s, scientists could only hypothesize that such a thing as the spliceosome existed. Now, thanks in large part to the pioneering work of UCSF's Christine Guthrie — work done in "lowly" yeast — scientists not only know of its existence, but much about its mechanism of action as well.

"I had a strong intuitive belief that yeast had a spliceosome," says Guthrie of those early days. A decade of work, characterized as "a long, hard slog," proved the soundness of her intuition. And it became clear during those years that the spliceosome matters.

Scientists now estimate that 50 percent of inherited diseases result from missteps during its activation, meaning that insights into the molecular instruments that make up the machine could lead to new diagnostics and more sophisticated therapies.

All this was far from Guthrie's mind when she took up the challenge that yeast spliceosomes even existed, much less that they were a worthy model. The first proof came when Guthrie's team discovered the genes that code for molecular tools in the spliceosome; they are known as small nuclear RNAs, or snRNAs. Geneticists had found such genes in the cells of mammals and even sketched out a role for them as guides to the splicing process, proposing that they lined up at sites along the RNA that need to be snipped out. But while scientists were unclear on the details, they seemed certain that these molecular landmarks would not be found in yeast.

Guthrie and her colleagues not only demonstrated that these genes exist in yeast, but also that, without just one of them, yeast cells died. Similarly, they determined that changing just one nucleotide in a small nuclear RNA, or changing its partner in the section of messenger RNA that was to be deleted, stopped splicing cold. And as an exclamation point, they then paired the two mutants and restored splicing.

"We were able to go quite far in establishing a mechanism," says Guthrie. "The discovery proved a terrific boon because of the difficulty of doing genetic manipulation in mammals." Now, Guthrie and her laboratory colleagues, established in their new quarters at UCSF Mission Bay, are helping to illuminate the role of the spliceosome in what is called regulated "alternative splicing" — a process that helps to explain how our 35,000 genes can code for the two to three times as many proteins than we possess.

That is, in another symbol of evolution's versatility, the spliceosome sometimes skips over part of the coding region of a gene, thereby altering the blueprint of the subsequent protein. As a consequence, a parent gene may order up several or more protein offspring with duties in places as varied as the brain and the skeletal system.

In mammals, alternative splicing requires so-called SR proteins. Guthrie has demonstrated that a simpler version of this class of proteins also exists in her model system. Mammalian geneticists have generally assumed that this was not the case.

Where will the applicability of yeast genetics to their human counterpart ultimately end? Yeast, after all, has 6,000 genes in one cell; humans have some 35,000 genes in each of 100 trillion cells.

Moreover, yeast splices only a subset of transcribed genes, while mammals splice every transcript at multiple places. Still, as Guthrie explains, there is much to learn. Why, for example, do yeast even have introns? Why do they splice only occasionally? Perhaps, Guthrie theorizes, yeasts splice when they need to regulate specific proteins in response to varying environmental conditions, such as heat, cold or infectious agents.

It was her curiosity about this possibility that now has her back at the lab bench where she and each of the fifteen members of her team are testing the impact of their favorite gene or environmental insult on yeast splicing.

They have been aided in these experiments by their custom-made version of the latest jewels of genetics, the microarray, or "gene chip," which allows scientists to see (on a glass microscopic slide) which genes are being expressed and, in this case, which are spliced.

The preliminary results? Promising, she says, grinning. And after 25 years of seminal discoveries, Guthrie is not someone to bet against.

Source: Jennifer O'Brien

Last updated January 28, 2005

Christine Guthrie

(From left) Mette Lund, Wendy Gilbert, Christine Guthrie and Tamara Brenner. Photo by Robert Foothorap.